Such chambers are constituted by an envelope inside which electromagnetically absorbent blocks are disposed that form the absorbent walls of the chamber. For a chamber of the anechoic type, all of the walls are covered in electromagnetically absorbent materials, whereas for a chamber of the semi-anechoic type, the floor does not include electromagnetically absorbent blocks (see FR-A-2 631 452, FR-A-2 623 288, JP-A-04 169 865).
Such enclosures lined fully or partially with blocks that absorb electromagnetic waves are commonly used for testing antennas or for measuring the levels of radiation from equipment over a given frequency range, and this applies to various technical fields (motor vehicles, satellites, telephones, etc. . . . ), with such measurements making it possible, for example, to verify that such-and-such specific low-current equipment operates properly even when in a high level of electromagnetic field at given frequencies corresponding to the type of equipment in question.
The positioning of antennas in such an installation is conventionally organized by using a carriage on which a vertical mast is mounted that supports an antenna (or sometimes two antennas) with the option of adjusting the height thereof.
Such a known positioning device is shown in FIG. 1.
That figure shows a measurement chamber 10 (in longitudinal section) constituted by a rectangular parallelepiped enclosure 11 having electromagnetically absorbent blocks disposed therein (e.g. absorbent cones 20 and absorbent angle members 21) that constitute the absorbent walls 12 that define the inside volume 40 of the chamber. In the present case, a portion of the floor 13 is free from absorbent blocks in order to provide a plane along which the carriage 14 can move. The carriage 14 caries a vertical mast 15 on which there slides a support 16 whose height is adjustable by means of a system comprising a motor 17 and a halyard 18, as represented diagrammatically by arrow 30, and on which a coaxial connection 19 is fastened. A wire-type antenna 50 is mounted on the support 16 and the position of the antenna within the inside space 40 is thus appropriately adjusted for the purpose of a particular test or measurement.
When it is desired to change the antenna in order to perform a test or a measurement in another frequency range, e.g. using a microwave horn, it is necessary to stop the current test, to penetrate into the chamber, to remove the antenna from the mast, and to install another antenna that is compatible with the new frequency range in which it is desired to work, and finally to reposition the new antenna in the desired location.
Those various operations require a considerable amount of time (in general at least one hour), thereby greatly increasing the cost of the tests or measurements performed. Also, in an anechoic measurement chamber, it is necessary to remove absorbent blocks in order to gain access to the carriage.
To avoid such constraints, proposals have been made to modify the structure of the above device so as to make it possible to mount two antennas on the mast of the carriage. However, in practice, it is not possible to leave both antennas simultaneously in place since the unused antenna constitutes a source of non-negligible disturbance to the electromagnetic field. It is then necessary for the operator to go back into the chamber and remove the antenna that is not in use, and such movement naturally disturbs any equilibrium that may previously have been established in the chamber.